This invention is directed to a current sensor capable of sensing unbalanced current in AC load conductors together with a detector circuit energized by the sensor so as to form a ground fault sensing and signaling apparatus.
Ordinary residential and industrial electric circuits are routinely protected against excessive current loads so that the circuit is interrupted before there is damage to the wiring or to the structure in which they are installed; however such circuits are usually unprotected against high resistance leakage paths which have a sufficient resistance to limit the current to a value less than that required to actuate the excess current circuit protection equipment.
Particularly in medical equipment and also wherever a ground path through the person is possible, there are health hazards from such shock. The hazards and other criteria are discussed in greater detail in my prior U.S. Pat. No. 3,683,302, the entire disclosure of which is incorporated herein by this reference.
Particularly in hospital situations with equipment containing electrical devices being connected to a patient is the hazard acute. When the patient's skin resistance is broken, the internal body resistance through the heart muscle is very low so that very low currents can cause severe physiological reactions.
Differential current transformers of the general type as this invention are largely used in commercial ground fault circuit interrupter equipment as ground fault sensors; however due to apparent limitations in the art, these sensor devices are limited to milliampere current resolution, thus unable to detect microampere current levels. The aforementioned limitations are considered to be due to the power conductor geometry problems which result in excessive leakage flux within the sensor means and inadequate shielding from external electrostatic and magnetic fields, either of which may cause an error signal to appear at the sensor output terminals. This invention provides a means for overcoming the limitations of other known current sensors by the novel arrangement of the power-carrying conductors, improved shielding means, and improved design of the detector coil including impedance matching.